If Raft Racers is what I think it is (a pair of water slides, one yellow and one blue, with an s-bend type layout?), then I've taken some measurements of it on Google Earth, and it on its own occupies an area of around 2,000 square metres, or roughly 34m*67m.
If you rip out what appears to be the Mole In One Adventure Golf as well, then the area goes up to 2,500 square metres if you go right up to Sky Rider.
If you rip out Sky Rider as well as those two, you have an area of nearly 4,000 square metres to play with, if you don't sacrifice any of the buildings in Imagination.
For some idea, this isn't a huge area, but could accommodate a family coaster of some sort, I reckon.
I'll look at some types Merlin might possibly go for and compare them to the given area.
Firstly, they could go for a Zierer Force 281, similar to Flying Eagle at Legoland Billund. This would fit onto the area, occupying a plot of around 1,500 square metres or roughly 31m*61m.
They could also go for a B&M Family Inverted Coaster. The Legoland division seems very enmeshed with B&M at the moment, what with various family inverts and family wing coasters in their new Chinese parks as well as the upcoming family wing coaster at Legoland Deutschland. With Chessington building a family wing coaster just up the road, I reckon a family invert would be more likely at LLW, of the 2 B&M models Legolands are currently installing en masse. Looking at the prototype model at Happy Valley Shanghai, it might be a bit too big for the area of Raft Racers alone, but could be doable if Sky Rider went as well, as it takes up an area of slightly over 4,000 square metres, or roughly 55m*85m. Lego could also do a custom layout to more bespokely fit the area if they wanted.